1. Field of the Invention
The present invention relates to apparatus and method for evaluating earth formations traversed by a well borehole and in particular to formation sampling and testing.
2. Description of the Related Art
Exploration and production of hydrocarbons require accurate and precise measurements of earth formations, which may contain reservoirs of the hydrocarbons. Accurate and precise measurements are important to enable efficient use of exploration and production resources.
Well logging is a technique used to perform measurements of an earth formation from within a borehole penetrating the formation. In well logging, a logging instrument or tool is conveyed through the borehole. The logging instrument performs the measurements from within the borehole at various depths. The measurements are associated with the depth at which the measurements were performed to create a log. In one embodiment, a wireline is used to support the logging instrument and to transmit measurements to the surface of the earth for processing and recording.
Many types of measurements can be made of the earth formation. In one type of measurement, a formation tester extracts a sample of a fluid from the formation. The fluid is then analyzed to determine a property of the fluid. In prior art tools, the fluid is illuminated with light and light reflected from the sample or transmitted through the sample is measured to determine the property. The light response is detected and converted to an electrical signal using a photodetector. Unfortunately, the environment deep within the borehole can have temperatures high enough to cause typical photodetectors, such as photodiodes, to respond poorly or fail entirely.
Therefore, what are needed are techniques to test a formation fluid and provide an accurate response in the high temperatures of a borehole environment.